Positron emission tomography (PET), which has been attracting attention as being effective for early diagnosis of cancer, is a test method of administering compounds labeled with a tiny amount of positron emitting radionuclides and detecting annihilation radiations emitted from inside the body, thereby imaging metabolic functions such as sugar metabolism to examine the presence or absence and the degree of diseases. PET devices for practicing such a method have been put to practical use.
The principle of PET is as follows: A positron emitted from a positron emitting radionuclide by positron decay annihilates with an adjacent electron to produce a pair of 511-keV annihilation radiations, which are measured by a pair of radiation detectors according to the principle of coincidence counting. As a result, the position where the nuclide exists can be located to fall on a line segment (line of response) connecting the pair of detectors. An axis extending from the head to the feet of a patient who is a subject to be examined will be defined as a body axis. The distribution of nuclides on a plane intersecting perpendicularly with the body axis is determined by two-dimensional image reconstruction from data on lines of response measured in various directions on the plane.
To increase the sensitivity of a PET device, as illustrated in FIG. 1, a detector ring 12 of cylindrical shape upon which a large number of PET detectors 10 are disposed in a circumferential direction and an axial direction needs to be disposed in a tunnel-like configuration to increase the measurement solid angle, and the distribution of nuclides in the tunnel needs to be determined by three-dimensional image reconstruction. The long tunnel-like patient port, however, increases the psychological stress of the patient 8 on the bed 6 under examination, and interferes with external access to the patient 8 (for example, the irradiation of an affected area of the patient 8 with a radiation beam for cancer treatment which is a main purpose of the present invention). Here, the detector ring 12 mostly has a perfect circular shape. The PET detectors 10 are stacked in a direction perpendicular to the sections of the detector ring 12.
Under the circumstances, the applicants have proposed an open PET device (also referred to as OpenPET) which includes, as illustrated in FIG. 2, a plurality (two, in FIG. 2) of split detector rings 12A and 12B spaced apart in the direction of the body axis and has a physically opened field of view (also referred to as an open field of view) (Patent Literature 1).
The open PET device enables PET diagnosis during treatment and whole-body simultaneous imaging which have not been possible by conventional PET devices. Applications to real-time PET/CT are also possible. Specifically, a treatment can be administered to the open field of view through a gap 12C between the detector rings 12A and 12B. Take a radiation cancer treatment for example. The open PET device can check the cancer position during irradiation with a radiation treatment beam, or visualize the irradiation field of the radiation treatment beam in real time. However, since the detector ring is divided into a plurality of rings and the detector rings need to cover both ends of the irradiation field, the number of detectors increases and the configuration becomes complicated. There is also a problem of limited access directions.
FIG. 3 shows an example of a heavy particle beam, proton beam, or other particle beam cancer treatment device as an example of a device that uses radiations for cancer treatment. This device includes two ports such as a horizontal irradiation port 20X and a vertical irradiation port 20Y. Some devices are single field irradiation devices which include only a horizontal irradiation port or a vertical irradiation port. The irradiation port(s) is/are not always fixed. Some devices include a rotating gantry which is configured to rotate about a patient 8. The rotating gantry type is predominant of proton beam cancer treatment devices in particular. As shown in the diagram, the axis along a horizontal irradiation treatment beam 22X will be defined as X-axis, the axis along a vertical irradiation treatment beam 22Y will be defined as Y-axis, and an axis orthogonal to the Y- and X-axes will be defined as Z-axis. The Z-axis usually coincides with an axis in the direction of the body axis of the patient 8.
In such a particle beam therapy irradiation device, PET measurement during irradiation is needed to check an irradiation field 24 in the body of the patient 8.
As a method for combining a heavy particle irradiation device with PET, Non-Patent Literature 1 describes that the detector ring 12 is inclined to preserve an open space having a width C as shown in FIG. 4. As shown to the upper left of FIG. 4, the detector ring 12 has a perfect circular shape. The PET detectors 10 are stacked in a direction perpendicular to the sections of the detector ring 12. The internal space of the detector ring thus has an elliptical shape when viewed in a direction perpendicular to the Z-axis (the left of the diagram).